The disclosure of Japanese Patent Application No. 2000-024632 filed on Feb. 2, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to an art for evaluating the specific surface area of electrode catalysts fixed to fuel cell electrodes, an art for evaluating performance of fuel cells, an electrode catalyst evaluated according to the arts, and a fuel cell having the electrode catalyst.
2. Description of Related Art
The performance of electrode catalysts is one of the factors crucial for the performance of fuel cells, and a number of arts for evaluating performance of electrode catalysts have been suggested. For example, there is known a process wherein a fuel cell is formed and then discharged for measurement of its output voltage or a cyclic voltammetry process (CV process) wherein an electrode catalyst is soaked in an electrolytic solution and the voltage is swept for measurement of current. Both of these processes electrochemically perform evaluation by supplying electricity to electrode catalysts.
A fuel cell, which has been studied and developed earnestly in recent years, is a stack of a plurality of unit cells. The unit cell is formed by having a membrane-electrode assembly (MEA) sandwiched between separators. In general, if a fuel cell as a stack of unit cells is expected to demonstrate desired performance, each of MEA""s is required to have performance exceeding a desired level. Accordingly, if an electrode catalyst is to be evaluated after formation of a fuel cell, it is necessary to evaluate MEA""s individually. Because the output current from the respective MEA""s is negligible, the necessity to pay close attention to adhesion of metal ions makes the process of measurement troublesome. Furthermore, the evaluation of an electrode catalyst is possible only after formation of MEA""s, which require troublesome work when being formed and can be adjusted in quest of desired characteristics only with great difficulty.
In general, an electrode catalyst for fuel cells, which has a noble metal such as platinum carried on a carbon carrier, is fixed onto an electrolytic membrane together with an electrolyte binder, thus forming an MEA. The carbon carrier generally has a particle size of 20 nm to 100 nm and is characterized in that carbon atoms (primary particles) securely adhere to one another to form a secondary particle (agglomerate). Platinum has a particle size of about 2 nm to 3 nm. On the other hand, since the particle size of the electrolytic binder is larger than the distance between the primary particles, the electrolytic binder cannot enter gaps between the primary particles. Consequently, part of the platinum cannot contribute to chemical reaction. According to the CV process wherein an electrode catalyst is soaked in an electrolytic solution for measurement, the particle size of the electrolytic solution used herein is smaller than the distance between primary particles, and the electrolytic solution also comes into contact with platinum that is out of contact with an electrolytic binder (platinum that is irrelevant to catalytic reaction). Accordingly, the performance of the electrode catalyst is evaluated, taking into account the platinum that is irrelevant to the reaction. As a result, it is difficult to evaluate performance of electrode catalysts (performance of fuel cells) with sufficient accuracy.
The invention has been made to solve the aforementioned problems. It is an object of the invention to simply and accurately evaluate performance of fuel cells. It is also an object of the invention to simply and accurately evaluate the specific surface area of electrode catalysts. Furthermore, it is an object of the invention to provide a high-performance electrode catalyst and a high-performance fuel cell by using the arts for evaluation. It is also an object of the invention to simply and accurately evaluate performance of gaseous-reaction electrodes.
To solve the aforementioned problems, in a first aspect of the invention, there is provided a device for evaluating performance of fuel cells. The device includes a container that holds an electrode catalyst used in a fuel cell, a supplier that supplies a predetermined amount of carbon monoxide to the container, and a detector that detects an amount of carbon monoxide discharged from the container. The amount of carbon monoxide adsorbed by the electrode catalyst is determined based on the amount of carbon monoxide supplied by the supplier and the amount of carbon monoxide detected by the detector. The output voltage of the fuel cell is calculated based on the determined adsorption amount.
According to the aforementioned aspect, it is possible to simply and accurately evaluate performance of electrodes based on the adsorption amount of carbon monoxide.
In another aspect of the invention, there is provided a device for evaluating specific surface area of electrode catalysts for fuel cells. The device includes a container that holds an electrode catalyst used in a fuel cell, a reducing agent supplier that supplies a reducing agent, which reduces the electrode catalyst within an operating temperature range of the fuel cell, to the container, a carbon monoxide supplier that supplies a predetermined amount of carbon monoxide to the container after reduction has been carried out by the reducing agent, and a detector that detects an amount of carbon monoxide discharged from the container. The amount of carbon monoxide adsorbed by the electrode catalyst is determined based on the amount of carbon monoxide supplied by the supplier and the amount of carbon monoxide detected by the detector. The specific surface area of the electrode catalyst is calculated based on the determined adsorption amount.
According to this aspect, the reducing agent supplier can prevent the phenomenon of sintering from occurring in the electrode catalyst. Also, the specific surface area of electrode catalysts for fuel cells can be measured easily and precisely based on the adsorption amount of carbon monoxide.
In still another aspect of the invention, there is provided a method for evaluating performance of fuel cells. The method includes the steps of supplying carbon monoxide to a container holding an electrode catalyst used in a fuel cell and determining an amount of carbon monoxide adsorbed by the electrode catalyst. The output voltage of the fuel cell is calculated based on the calculated adsorption amount.
According to the aforementioned aspect, since the output voltage of the fuel cell is calculated based on the adsorption amount of carbon monoxide, it is possible to simply and accurately evaluate performance of the fuel cell.
In still another aspect of the invention, there is provided a method for evaluating specific surface area of electrode catalysts for fuel cells. The method includes the steps of supplying hydrogen gas to a container holding an electrode catalyst used in a fuel cell and reducing the electrode catalyst within an operating temperature range of the fuel cell by means of the hydrogen gas. After the reduction has been completed, carbon monoxide is supplied to the container. Then, the amount of carbon monoxide adsorbed by the electrode catalyst is determined, and the specific surface area of the electrode catalyst is calculated based on the determined adsorption amount.
According to this embodiment, the phenomenon of sintering can be prevented from occurring in the electrode catalyst through reduction thereof. Also, since the specific surface area of the electrode catalyst is determined from the adsorption amount of carbon monoxide, the specific surface area of the electrode catalyst can be evaluated simply and accurately.
In still another aspect of the invention, there is provided a method for manufacturing electrode catalysts for fuel cells. The method includes the steps of preparing a carbon carrier having a specific surface area of 1000 m2/g or more and having platinum carried on the carbon carrier with a predetermined platinum-carrying density.
The performance of the electrode catalyst is enhanced as its specific surface area increases. The performance converges if the specific surface area of the carbon carrier reaches 100 m2/g or more. Therefore, the aforementioned aspect makes it possible to manufacture high-performance electrode catalysts for fuel cells.